1275cc A plus engine carb upgrade.

[metro 1275]

Hello,

can a standard 1275cc A plus engine handle 2 HIF44 carbs or a webber 45?


I have just finished rebuilding this 1275cc A plus engine with high quality parts.

Thank you,

[AC43]

metro 1275 said:

Hello,

can a standard 1275cc A plus engine handle 2 HIF44 carbs or a webber 45?


I have just finished rebuilding this 1275cc A plus engine with high quality parts.

Thank you,

In the 80's I stuck a Dellorto 40 on a 1275 A Series. Not sure what differences there are between that engine and an A Plus but if the heads and ports are the same there should be no problem.

[carinaman]

David Vizard's book on tuning the A-Series is on eBay from £20.

[aka_kerrly]

carinaman said:

David Vizard's book on tuning the A-Series is on eBay from £20.

you mean the A series bible

Yes twin 40s can work. One of my neighbours used to have a Midget on twin 40s mounted on a swan neck intake.

OP il have a look in my book collection as a may have a mini tuning book from when i built one for an ex gf years ago, if i find it you're more than welcome to it.

[KEITHOA]

A 45 would be way to big for a standard 1275
It would prob need 36mm chokes but the air speed will be so slow when the throttle opens it would prob struggle at lower rpm
A 40 would be a better bet but still need small chokes
A single su is good for around 100hp
The Weber wont as good on fuel either

Twin su would work ok

Edited by KEITHOA on Wednesday 29th May 18:03

[metro 1275]

Thankyou, that would be helpfull.

[carinaman]

There's a pair of 1 & 1/4 inch SUs on a manifold for a Mini on eBay for £30 inc. postage with a day to go.

[OnTheEdge]

Twin HIF44's would be massive overkill for a standard 1275, even a full race motor wouldn't use their potential. A DCOE45 is also too big for your engine, and will be even harder to tune than oversized SUs.

Don't bother with the original twin SU manifolds either, they are surprisingly crap (much better aftermarket manifolds available). A single HIF44 will be good for 100bhp at least so will give plenty of scope for tuning the engine. The cylinder head is the fundamental power bottleneck in these engines, and until that is fixed bigger carbs will just cause problems.

[stevieturbo]

metro 1275 said:

Hello,

can a standard 1275cc A plus engine handle 2 HIF44 carbs or a webber 45?


I have just finished rebuilding this 1275cc A plus engine with high quality parts.

Thank you,

As others have said, total waste of time anything other than a single 44 for a 1275..even a lot of modified ones.

If you're after more power...it'll be the usual head work, cam, exhaust, intake etc etc. It's always a lot of work for meagre gains though n/a but good improvements over standard can definitely be achieved.

[DVandrews]

There is a lot you can do to the regular twin 1 1/4 Sus to improve the airflow while maintaining decent airspeed, squaring of the bridges, radiusing the edge of the pistons and feathering of the throttle plates and spindles, the HIFs would be overkill, a single 40 on a long manifold is also a good solution, split Weber’s give better mixture distribution but are expensive.

Dave

[cmsapms]

My 2p

Back in the 80s I had a Mini with an overbored (74mm) A series. This had a head self-modified along the lines of the Vizard Bible, Piper 300 cam and 1.5:1 rockers. Initially I used a single SU HIF6 carb which made the engine feel slightly strangled. I then made an inlet manifold from a couple of short pieces of scaffold tube to allow me to add another HIF6. This worked remarkably well, and was very tractable and powerful. I didn't get the opportunity to put it on a rolling road, as the car was stolen, so it was never optimised.

I set up a camera and a mirror under the bonnet to see what the piston was doing at various engine speeds/loads, particularly WOT at peak power RPM. I did this for both single and twin setups. The single carb's piston was fully "up" some way short of peak power, whilst the twin setup's pistons never got more than two thirds of the way to fully "up".

Therefore I'd say that two HIF44s (same size as HIF6) would be complete overkill on the spec of engine you are proposing. I'd say twin 1.1/4s would be ideal if you paid attention to making them work correctly and put them on an efficient manifold.

[PeterBurgess]

Interesting observations Paul, thanks for sharing them, info tucked away for future reference. I
go along with the idea that SU carbs are correctly sized when the pistons hit the top around 3/4 full power rpm.

[Lotobear]

Peter,

Can I pick your brains? (apologies for thread hijack but I think the OP is now informed).

I have a Mini with an MG Metro lump installed (the version with the standard valve head), standard bore 1275. It has a Maniflow 3:1 and a twin box RC40 exhaust and K@N cone filter with a HIF44 carb.

It has a misfire when revved and seems to hunt at idle. I've looked at the needle and its a BDR, the spring is unknown.

Would I be better off with a BDK and red spring?

[PeterBurgess]

I would try BDL and red spring see if the miss was from being rich, if worse try the BDR. set the jet 85 thou down from the bridge. To be honest you would be best going to a rolling road and trying the different needles and timing settings.